Cyclic confining pressure and rock permeability: Mechanical compaction or fines migration

Area of Science:

• Petroleum Engineering
• Geology
• Material Science

Background:

• Apparent rock permeability decreases under cyclic loading, attributed mainly to irreversible deformations.
• Current methods for permeability measurement under cyclic loading overlook the influence of fluid-borne colloids.
• Colloidal migration's impact on pore throat clogging and subsequent permeability reduction is often neglected in porous rock studies.

Purpose of the Study:

• To provide experimental evidence that colloid migration, in addition to mechanical compaction, causes permeability reduction during cyclic loading.
• To differentiate the contributions of mechanical compaction and colloid migration to apparent permeability changes.
• To highlight the significance of colloid migration in routine core tests.

Main Methods:

• Developed a novel methodology involving purging core samples with a high-pressure gradient between loading/unloading cycles.
• Conducted multi-cycle confining pressure tests on rock core samples.
• Measured apparent permeability changes under varying gas flow directions and regimes.

Main Results:

• Apparent permeability of rock samples decreased by up to 20% due to colloid migration under cyclic loading.
• Permeability dynamics became unpredictable and showed weak dependence on confining pressure when flow direction/regime changed.
• Apparent permeability recovered to initial values in 3 out of 4 samples post-testing, confirming colloid migration's effect.

Conclusions:

• Colloid migration is a significant factor influencing apparent rock permeability under cyclic loading and gas injection.
• The proposed mechanism explains apparent permeability changes during gas injection and cyclic confining pressure.
• Colloid migration must be incorporated into core test analyses, including routine coreflooding and cyclic loading tests.